Anisotropic conductive films (ACFs) are commonly used in flat panel display driver integrated circuit (IC) bonding. A typical ACF bonding process includes a first attachment step in which the ACF is attached onto the electrodes of the panel glass or flexible printed circuit (FPC) board, a second step in which the driver IC bonding pads are aligned with the panel electrodes, and a third step in which pressure and heat are applied to the bonding pads to melt and cure the ACF within seconds. The conductive particles of the ACF provide anisotropic electrical conductivity between the panel electrodes and the driver IC.
The need for ultra-fine pitch ACFs increases dramatically as the use of high definition displays in electronic devices such as smart phones and electronic tablets become the market trend. However, as the pitch size decreases, the size of the electrodes must also become smaller and a higher concentration of conductive particles is needed to provide the required particle density on the connected electrodes to assure satisfactory electrical conductivity or impedance.
The conductive particles of a traditional ACF are typically randomly dispersed in the ACF. There is a limitation on the particle density of such a dispersion system due to X-Y conductivity. In many bonding processes using traditional ACFs, only a small fraction of conductive particles are captured on electrodes. Most of the particles are actually flushed out to the spacing area between electrodes and in some cases result in undesirable shorts in the X-Y plane of the ACF. In a fine pitch bonding application, the conductive particles density should be high enough to have an adequate number of conductive particles bonded on each bonding pad. However, the probability of a short circuit or undesirable high-conductivity in the insulating area between two bonding pads also increases due to the high density of conductive particles and the characteristics of random dispersion.
Fixed-array ACFs overcome some of the shortcomings of traditional ACFs. The conductive particles of a fixed-array ACF are arranged in a pre-determined array pattern. Fixed-array ACFs have been recognized as one of the most effective approaches for achieving high resolution connection of ultra-fine pitch ICs. For example, a minimum bonding area as small as about 300 to 400 μm2 and a minimum bonding space as narrow as 3 μm have been demonstrated with fixed-array ACFs having a conductive particle density of at least 30,000 pieces per mm2 (pcs/mm2). Some references which discuss fixed-array ACFs include, for example, Liang, R. C. et al., “Fixed-Array Anisotropic Conductive Film (FACF) for Ultra Fine Pitch Applications,” International Conference on Flexible and Printed Electronics (ICFPE) Proceedings, Paper S1-2-4, Hsinchu, Taiwan (2010); “Ultra Fine Pitch Anisotropic Conductive Film with Fixed Array of Conductive Particles,” IDW'10 Proceeding, p. 1909, Paper FMC4-4, Fukuoka, Japan (2010); “Ultra-Fine Pitch Fixed Array ACF,” Tech on Chinese (Mar. 1, 2011.); and U.S. Publication No. 2014/0141195 to Liang et al.
Fixed-array ACFs also have some drawbacks. In particular, attachment of the fixed-array ACF to the electrode substrate is not reproducible if the density of conductive particles of the fixed-array ACF is too high. A conductive particle density of, for example, 20,000 pcs/mm2 or more may impede the attachment of the fixed-array ACF to the electrode substrate because the surface of the fixed-array ACF is predominately covered by an array of non-tacky conductive particles. Moreover, due to the high concentration of conductive particles such as gold (Au), nickel (Ni) (i.e., >30,000 pcs/mm2) or other metalized polymer particles, the coloring of the fixed-array ACF may be relatively dark or brownish in color, and hazy. If the ACF is used in conjunction with light-emitting device such as a light emitting diode (LED) or an organic LED (OLED), then the dark color of the fixed-array ACF may negatively impact the light intensity and color purity emitted and/or reflected from the light-emitting device. Similar issues also exist if the fixed-array ACF is used in a light-transmitting or reflecting device such as a liquid-crystal display (LCD) display.
In one attempt to alleviate the deterioration in light intensity and color purity of the device a reflective pigment, such as titanium dioxide (TiO2), was added to the existing adhesive layer of the light-emitting device. However, adding reflective pigment to the existing adhesive layer does not improve reflectivity, light output, or color purity of the device unless the concentration of reflective pigment is higher than a certain threshold value. However, a high concentration of the reflective pigment in the adhesive may tend to result in a significant degradation in particle transfer efficiency and uniformity in the microfluidic particle transfer process as well as the easiness of the electrode attachment step. In contrast, light scattering within a thick adhesive layer containing a low concentration of reflective pigment may result in an improvement in the percentage of reflectance, but a driver IC or LED chip with a thicker bump height would be required. This will result in a trade-off in the cost, package thickness and the capture rate of the conductive particles on the electrodes and bumps to be connected. The capture rate is defined as the percentage of the ACF conductive particles captured on the overlapped bonding areas of the bumps and electrodes after the main bonding process. For a given ACF, the higher the capture rate, the lower the connection resistivity and the lower the risk of short circuit because of the less conductive particles in the space between electrodes. Accordingly, there exists need in the art for an improved fixed-array ACF having a high concentration of conductive particles that results in improved reflectivity, light output, and color purity characteristics without the trade-off in manufacturing yield, uniformity, and attachment or bonding properties of the ACF, and the capture rate of conductive particles and the reliability of the connected electrodes/chips after bonding when used in light-emitting and light-transmitting or reflective devices.